Reducing the electromagnetic emission is still a major concern in a wide field of applications including in particular electronic circuitries to be used in electromagnetic emission sensitive environments such as automotive applications and aviation applications. All electronic systems emit some unwanted radio frequency energy. Emitted energy may cause any unwanted interference in particular within tightly packed integrated circuits or electronic circuits arranged nearby. The emitted noise generally appears at sharp spectral peaks usually at the operating frequency of the device, and a few harmonics thereof. The problem of radio-frequency emission and interference caused thereby gets further worse in view of microprocessors and microcontrollers as well as communication interfaces required to operate at high frequencies in order to meet performance requirements and.
A system designer can use a spread-spectrum clock signal generator such as a spread-spectrum PLL to reduce interference by spreading the energy over a larger portion of the spectrum. For example, by changing the operating frequency up and down by a small amount (about 1%), a device running at e.g. hundreds of megahertz can spread its interference evenly over a few megahertz of spectrum, which drastically reduces the emitted energy at a given frequency. Frequency modulation of a clock signal is a well-known and efficient way to spread clock harmonics around a center frequency, thus reducing emitted narrow-band RF energy.
For instance US 2009/0135885 A1 suggests a spread spectrum clock generator which makes use of a random number generator, which controls a voltage-controlled oscillator, in a phase-locked loop (PLL) circuit. The U.S. Pat. No. 6,687,319 B1 suggests a use of a random number generator to randomly gate off individual clock cycles generated by a crystal oscillator. A reference clock signal is transmitted in parallel, which allows reconstructing the off-gated clock cycles. The clock signal with gated-off clock cycles is spectrum-spread and may be provided as a high power signal, whereas the reference clock signal is provided as low power signal causing less narrow-band emission.
Further examples of spread spectrum clock generators are suggested in “Dithered Timing Spread Spectrum Clock Generation for Reduction of Electromagnetic Radiated Emission from High-speed Digital System”, Jonghoon Kim et al, Electromagnetic Compatibility, 2002; “Spread Spectrum Clock Generator for reducing Electro-Magnetic Interference (EMI) Noise in LCD Driver IC”, Jaehong Ko et al, Circuits and Systems, 2007; and “A Synchronization-Free Spread Spectrum Clock Generation Technique for Automotive Applications”, Junfeng Zhou et al, IEEE Transactions On Electromagnetic Compatibility, Vol. 53, No. 1, February 2011. The spread spectrum clock generators suggested in the above mentioned scientific articles use selectable delay lines to shift edges of the clock signal in accordance with predefined functional relationships.
Some communication interfaces such as inter-chip communication protocols do not tolerate variations of the frequency of a clock signal such as those variations generated by the above mentioned spread spectrum clock generators. Hence, one or more additional unmodulated clock signal generators such as narrow-band PLLs are used to generate the clock signals for the inter-chip communication and/or peripherals connected through communication interfaces. The implementation of additional clock generator circuits is expensive with regard to the required die area and power to operate the additional clock generator circuits.